If you don’t take proper precautions, your car could be stolen—even without the thief having your keys. This is because modern car keys constantly transmit signals. According to cybersecurity experts, criminals can purchase a key similar to yours and clone your car’s access code to gain entry and start the engine.

So how can you protect yourself?

One simple, low-cost method is to wrap your keys in aluminum foil. While not perfect, it blocks the signal from being transmitted and received by unauthorized devices. Experts agree it’s an easy and effective temporary solution.

A more durable option is to use a Faraday bag, which you can buy online for just a few dollars. These specially designed pouches block electromagnetic signals and prevent your key’s information from being intercepted.

Cybersecurity expert Moshe Shlisel, CEO of GuardKnox Cyber Technologies and former developer for Israel’s missile defense systems, compares the key’s signal to a song. “If your car opens when it hears a specific tune, then anyone who knows the tune can unlock it,” he told BBC Mundo.

Aluminum foil or a Faraday bag creates a protective barrier, or “cell,” that stops these invisible “songs” from being captured and reused by thieves.